1. Field of the Invention
The present invention relates to a thread insert having a detachable tongue which is screwed into a hole formed in a work for providing a female thread on the inner surface of the hole.
2. Description of Related Art
Thread inserts are helical parts used for forming a female thread on the inner surface of a hole formed in a work which is made of a material having relatively low hardness such as plastics or aluminum.
FIGS. 20-22 illustrate an example of conventional thread inserts. This thread insert is formed by winding a metal wire in a helical shape, and a male screw 31 and a female screw 32 are respectively formed on the outer and the inner surfaces of the helical wire. At the front end of the helical wire, a first coil portion 33 having a smaller diameter is formed so that the thread insert can be smoothly screwed into a hole from the side of the first coil portion 33.
A detachable tongue 34 is formed at the front end of the first coil portion 33 so as to extend toward the center of the first coil portion 33, and a notch 35 is formed on the wire by cutting between the first coil portion 33 and the tongue 34 for facilitating the detachment of the tongue 34.
When the thread insert is used, the detachable tongue 34 is grasped by a tool (not shown) inserted through the thread insert, and the thread insert is screwed into a hole of a work by the tool. Then, the detachable tongue 34 is pushed by an another tool inserted through the screwed insert, and the tongue 34 is detached by breaking the wire at the notch 35. After detaching the tongue 34, it is possible to screw a bolt into the insert beyond the length of the insert.
In order to extract the insert screwed in the work, as shown in FIG. 23, a detachment tool 37 is pushed into the back end portion 36 of the thread insert so that a pair of cutting edges 39 of the tool 37 bite in the inner thread 32 of the insert. Then, the tool 37 is rotated so as to unscrew the insert from the work.
However, the above thread insert has the following drawbacks. That is, the above thread insert cannot be extracted unless the special detachment tool 37 having cutting edges is used. Furthermore, if the pushing force of the detachment tool 37 is too weak, the tool 37 does not form notches 38 having a sufficient depth, and it is not possible to rotate the thread insert by the tool 37. In contrast, if the pushing force of the detachment tool 37 is too strong, the tool 37 enlarges the diameter of the thread insert, and the risk will arise that the inner surface of the hole will be damaged. Should the inner surface of the hole be damaged, it becomes necessary to re-tap the hole in order to attach a new thread insert in the same hole, and the new thread insert must be larger than the original one if the inner diameter of the hole is enlarged by re-tapping. Therefore, the operation of the detachment tool 37 is not easy and requires the skill of an expert.
Additionally, because the notch 35 is formed on the front surface of the wire as shown in FIG. 21, when the tongue 34 is pushed by the tool inserted through the thread insert, a crack generates from the back surface 35a opposite to the notch 35. However, the back surface 35a is smooth, stress generated by pushing the tongue 34 does not concentrate on a single point in the wire, and breaking the wire requires large load.
Furthermore, because the back end 36 of the insert is flat, when the thread insert is unscrewed, the edge 36a of the back end 36 scrapes the inner surface of the hole. The scraping by the back edge 36a not only increases the rotational resistance of the insert, but also increases the risk that the inner surface of the hole will be damaged.